Chronic sleepiness is the greatest cause of functional impairment for people with narcolepsy, obstructive sleep apnea, and many other sleep disorders. Very little is known about the neurobiology of sleepiness, but the discovery that narcolepsy is due to orexin/hypocretin deficiency provides a unique opportunity to investigate the pathways that regulate behavioral state. We hypothesize that orexin promotes and stabilizes wakefulness by increasing the activity of specific arousal regions such as the locus coeruleus and tuberomammillary nucleus. The overall goal of our experiments is to define the physiology and essential neural pathways that underlie the sleepiness of mice with impaired orexin signaling. First, we will characterize in detail the sleepiness of orexin deficient mice to gain a better understanding of the behavioral changes produced by orexin deficiency. In addition to a thorough analysis of sleep/wake behavior, we will assess the ability of these mice to maintain vigilance as impaired vigilance is a sensitive and functionally important consequence of sleepiness. We will then use an innovative and powerful molecular technique to identify the brain regions through which orexin promotes and stabilizes wakefulness. Specifically, we will produce inducible orexin receptor knock-in mice that are born lacking both orexin receptors. We will microinject an adeno-associated viral vector containing Cre recombinase into specific brain regions of these mice to induce eutopic, focal expression of the orexin receptors. We then will examine the sleep/wake behavior and vigilance of these mice to determine which brain regions are sufficient for orexin's wake-promoting effects. Lastly, we will define the neurochemically specific populations through which orexin promotes and stabilizes wakefulness by crossing these inducible orexin receptor knock-in mice with transgenic mice expressing Cre only in the histaminergic, catecholaminergic, or cholinergic neurons. We predict that expression of orexin receptors in the tuberomammillary nucleus or other arousal regions will normalize the ability of these mice to maintain wakefulness. These experiments will define which neurochemically specific populations of neurons are sufficient for the normal maintenance of wakefulness and vigilance mediated by orexin, thus providing crucial insights into the cause of sleepiness in narcolepsy and other sleep disorders.